Heretofore, self propelled apparatus has been provided, such as shown in U.S. Pat. No. 4,603,516 for treating the outer surface of a pipeline or pipe as the apparatus moves along a pipeline while supported thereon. Treating the outer surface of the pipeline such as by cleaning removes loose rust, scale and dirt, to prepare the pipe for the subsequent application of a coating material on the outer surface of the pipeline to minimize corrosion and prolong the service life of the pipeline. In some instances, the pipeline may be wrapped with an outer lining material containing an inner coating material for contacting the outer cleaned surface of the pipeline. Abrasive particles, such as hard shot particles or grit, have been thrown by centrifugal force from an impeller wheel or the like against the outer surface of the pipeline for cleaning the pipe. Particularly where hard shot particles of a uniform size have been used a relatively smooth clean outer surface has been obtained on the pipeline.
It is desirable to have a roughness on the outer surface of the pipeline to provide a strong bond between the pipeline and coating. The roughness increases the surface area of the pipe in contact with the coating for "anchoring" the coating on the pipeline. Such an increased area bond is particularly needed upon an expansion of the pipeline resulting from pressure or temperature increases to maintain the bonding contact between the pipeline and coating. The greater the roughness, the stronger the bond between the pipeline and coating particularly for shearing stresses. An enclosed blast chamber or housing on a self propelled carriage travelling along the pipeline has been utilized previously but such carriages have been relatively complex for carrying blast wheels or impellers and for assembly on and disassembly off the pipeline.
It is desired under certain conditions to have cleaning nozzles which move in an arcuate path concentric to the pipe so that the nozzles maintain a constant radial distance from the outer surface of the pipeline during the cleaning operation. In this manner the velocity of the fluid and entrained particles striking the outer surface of the pipeline is generally uniform since a constant distance form the pipe is maintained. U.S. Pat. No. 4,953,496 dated Sep. 4, 1990 provides a cleaning apparatus in which wheeled nozzle carriages are mounted for arcuate movement along arcuate rings which are secured to mounted arms. The nozzle carriages are engaged by chains for oscillating movement relative to the arcuate rings with the nozzles maintaining a constant radial distance from the outer surface of the pipe. Thus, a relatively complex mounting is provided in the '496 patent in order to oscillate the nozzle carriages along an arcuate path relative to the arcuate rings.